The biogeographic and genomic signatures of dynamic river networks for terrestrial species in Amazonia

Amazonia contains Earth's largest freshwater basin, largest contiguous stretch of tropical forest, and most species-rich terrestrial biota on Earth. Rivers are key geographic features that drive diversification of the Amazonian biota, but they are also dynamic, which challenges their role as long-term barriers to dispersal and gene flow. The impacts of such river dynamics on organismal evolution have only recently been explored in detail. Here I examine biodiversity patterns and processes in Amazonia to elucidate how taxa diversify in the context of river network dynamics. I borrow the River Capture Hypothesis from ichthyology, and draw on evidence from speciation genomics, hybrid zones, and community assembly to demonstrate the effects of river network evolution on biodiversification. The idea is simple: populations of organisms whose dispersal is restricted by rivers become semi-isolated by rivers. Drift and selection against introgression drive divergence, but as rivers move, previously isolated populations come into secondary contact, facilitating lineage fusions or the migration of hybrid zones to other rivers. The basin's unique macroecological patterns and rich biota thus may have resulted from repeated divergences, lineage fusions, and range expansions around a network of non-stationary extrinsic barriers with variable results depending on the degree of intrinsic reproductive isolation that accumulates during this process. The evolutionary consequences of dynamic landscapes extend beyond Amazonia as "fission-fusion-fission" cycles modulate the diversification and spatial patterning of life on Earth in general.